Thursday, October 13, 2011

W1GHZ rover transverter for the 3400 MHz - EU band coverage

And finally, the 9cm band cheap rover W1GHZ transverter for the EU band coverage is also finished. As already mentioned in the 13cm transverter post, the band coverage with the original L.O. does not permit operation on the EU portion of the band, in this case 3400-3410 MHz. Some modifications were required and here we go, another easy and cheap way to start with the new band not spending the big bucks. Not yet in the proper housing but even without it no unwanted oscillations or products observed.

Looking from the left side there is a small power amplifier with the AH-102A and a peace of heatsink from the old PC switching power supply followed by the well known W1GHZ transverter board for the 9cm band with the 1/2" pipecap filters. As the transverter will be used at the Europe with the different 9cm band allocation the original 720 MHz local oscillator does not help in this case. Separate local oscillator, S53MV style, already used in the 13cm transverter will be used but with the different output frequency. At the end on the right side there is a so called "sequencer" taking care of switching power and everything that have to be switched in the transverter.

I will start from the oscillator this time. After the multiplying (5x) the original 720 MHz oscillator should give us the 3600 MHz signal on the transverter board after the pipecaps. Mixing with the 144 MHz (LSB) we should reach 3456 MHz. If we want to use this idea in the EU we should have a 200 MHz I.F. radio which is not convenient at all. Going back to the well proved practice, the S53MV oscillator used in the ZIF microwave radios was a cheap and flexible solution offering many multiplying schemes to reach required frequency. For the I.F. I choose the 70cm band where we have 10 MHz (from 430-440 MHz) comparing to the 2m band where only 2 MHz (144-146 MHz) is available for conversion. Looking through my crystal stock (this is where all your "odd" crystals comes handy) I found the one marked 20.585 MHz. After the chain of multipliers on the oscillator board the output frequency was 741 MHz. The exact frequency was tuned with the coil and the trimmer capacitor near the crystal. (The multiplier scheme was 20.58333 x 3 x 3 x 2 x 2) More that 10dBm of the stabile signal was enough for the next multiplier MMIC stage on the transverter board.

At this stage the transverter PCB was not populated completely, just the multiplier parts required and all pipe caps. To avoid an extra multiplier, as on the 13cm modified rover transverter, I decide to use the original pipe caps tuned to the 4th instead of original 5th harmonic frequency. Idea was to multiply oscillator 741 Mhz four times to 2964 Mhz. Mixing with the I.F. of 436 MHz we are reaching the 3400 MHz band. No need for the fancy measuring equipment, all can be done just with the "LNB diode power meter" if you previously measured correctly 741 MHz from your L.O. So how does it work? First, be sure to have all pipe caps with the screws completely inside touching the PCB inside the filter.10dBm at the input is enough to drive the ERA-2 (A4 marked first MMIC) in the saturation producing reach harmonics. Backing out the screw from the first filter and at the same time measuring the power just after the second ERA-2 (marked A5) you will notice just after the few turns (2 or 3) a peak in the power meter. This is obviously the 3rd harmonic at 2223 MHz as this pipe caps are too small to tune 2nd harmonic (1482 MHz). Backing out the screw will bring us to the next peak which should be 4th harmonic at 2964 MHz, the one we are looking for. So you can stop here, or if you want to see what this simple multiplier is capable of you can continue with the screw reaching the 5th harmonic at the 3705 MHz. I even reach the 6th harmonic at 4446 MHz if I remember well.

The next thing is to replace the measuring points and to connect the power meter at the L.O. test point after the second pipe cap. Tune the second pipe cap filter backing the screw to get maximum power. You can fine tune now the first filter to the maximum, but practically the MMIC isolation should be high enough preventing the influence of the filters to each other. At this point 7-10 dBm of clean L.O. signal is available, depending how patient with the screws you are. After tighten the screws I like to secure them with some extra nail varnish.
As you notice, I did make some bridge where the mixer should be installed allowing this way to tune also the filter after the mixer to the maximum at the L.O. frequency of 2964 MHz, of course measuring the power just after the filter. The I.L of the filter should not be higher than 2-3 dB so you should easy measure some 7 dbm just after the filter. Do not forget to remove the bridge :-)
Now it is time to populate the rest of the PCB and to complete the transverter. This time I did follow the project and I didn't swap the MMICs. The only thing that I change are the bias resistors. I don't like to have different RX and TX power supply so I convert everything to the 9V power supply, even the multiplier MMICs are 9V powered. Anyhow I think that the MGA-86576 has the odd value at the original design, not according the data specs.

After we have all parts on the board we can finally tune all together. With L.O. connected and rx side powered up it remain to bring the pipe cap filter after the mixer from previously tuned 2964 MHz to required 3400 MHz by backing the screw (several turns). Approaching the 3400 MHz the noise will also come out louder on the IF radio. The easy way to do that is just to listen any signal on the band :-). Of course, this is not 20mtrs band and it is not crowded, so it is handy to make some oscillator making the "noise" on the 9cm band. The easiest way is to use canned 40 MHz oscillator. The 85th harmonic should be heard with no problem!! Adding a LNB diode on the oscillator output pin, can even improve generating of the harmonics. I make the comb oscillator with the 16 DIL socket, so different canned oscillator can be used. 100 MHz oscillator can be handy for 9cm band as well. It is smart to add the 5V regulator on the sam board, just to stay on the safe side with all wires on the bench during the test and tuning faze.

Tuning the signal to the maximum, listening the radio is not the perfect way but can give us good idea how many turns we have to back the screw out from the pipe cap. Now we should connect the power meter to the TX SMA connector and apply some signal on the I.F. port. Not more then 1 mW on the 436 MHz is required to have 3400 MHz out. Do not forget to power the TX side of the transverter :-). Backing the screw from the pipe cap just before the ERA-5 should give us some output, bringing the screw at the same point as previous pipe cap. Fine tune the both pipe caps for the maximum output power. At the same time this will result the best receiving signal. I reach the 25 mW of the power on the 3400 MHz. Most probably, some more mW can be obtained with the careful tuning but I was lazy to play with it because I had in mind the next amplifying stage.

The RX side performed quite well, the sensitivity is just enough for the rover type transverter. TX part is stabile, no oscillations noticed. You can notice on the picture above that the screws head are just above the nuts, that's because when I tune the filter to the required frequency I like to cut excess length of the screw. Like that the complete transverter occupy less space and lower profile housing can be used. One important thing: before soldering the pipe caps, make the plan how you can do it easily. It is not the same which pipe cap will be soldered first. Try to visualize the job and you will see that there is a place just between the pipe caps that is very narrow and you can not access it easily. So try to make the best sequence and order in soldering the pipe caps to avoid this problem.
Using only 25 mW of the power can result in some qsos from the hill portable location, or the qsos with the big guns but at least 200 mW should be nice to have. Anyhow, half of that power we are losing trough the relays, connectors, coax, swr, coax to feed adapter etc.

Looking through my microwave surplus to find some quick and dirty amplifier bring me to the already proved design using the AH-102A MMIC. I knew that the data sheet said that this component can work up to 3 GHz, but it did not cost me nothing to try this MMIC on 3.4 GHz. The board with the MMIC was cut out from the old 2.1 GHz equipment together with bias network and input/output capacitors. First test did not bring my expectations to reach at least 200mW. On the 1.3 and 2.3 GHz this MMIC easily produce some 450 to 500 mW of power. Trying to match the input with the trimmer capacitor did not bring any improvement and the maximum of 150 mW out with 25mW drive was the result. Yeah, I did the tests with the 5V power supply, and going back to the data sheet I found that the power supply should be 9V!! This is why I change all the power supply in my transverter to 9V at the first, having in mind this amplifier. Applying 9V to the amplifier bring the smile on my face, 250 mW output or 10dB of the gain on 3.4 GHz. This was much closer to my expectations and no further tuning with the in/out SMD capacitors was done. Finally, a piece of heat sink from the old PC-switching power supply was prepared with the MMIC board attached allowing long TX sessions.

This is the so called sequencer that I am using in most of my transverters. Just a few words about it, because this one deserve a separate posting, maybe in the future. On the PCB there is IF TX-RX switching with the attenuator to reduce the IF radio power to 1dBm. On this place I am using the Omron type relay (black). The blue one is used for the power (DC) switching. Obviously there is a difference in the inner design where the blue type has a very poor isolation between the contacts. The Omron type is probably designed more likely as coaxial relay where much better isolation was achieved. On the same board there is a RF sensing switch, PTT possibility and NE555 hold/timer. The same board is driving also the output coaxial relay and at the same time switching the DC power supply with many options.

As complete project was cheap and easy I decide to use the same approach for the antenna. Cheap WA5VJB 2-11 GHz PCB Log.per antenna was placed in the focus of the prime feed dish 90 cm diameter. I did not care about the ilumination efficiency for the moment, but I found later on the web that the hole arangement is not so bad. Anyway this is rover, cheap and easy. For the first test I just hookup the transverter board with the L.O. on the FT-817nd tuned to the 70cm band. At this step I just want to check the receiving part, so no coaxial relay was used. A few SMA to N jumpers and a peace (2m) of not so lossy LMR-400 was connected to the LPA. After the tuning and all tests the comercial coaxial relay with SMA connector was attached to the unit. Here is the result:

Despite the fact that nothing was screened, the frequency was quite stabile without noticeable drift. Screening just the L.O. will improve the stability much more. Just one active amplifier stage on the receiving side gave us also surprisingly loud signal, much louder than expected. To give you some idea what I was listening on the video, I can tell you that the beacon was running not more than 500mW (probably less) into the omnidirectional waveguide slot antenna distanced 66 km away. Estimated 4 double slot antenna gain was 5dBi with (+/- 3dB) omnidirectional pattern H plane. With no clear line of sight and the path loss of the 200 dB the terrain slope was not good (what can be seen on the picture bellow) for the microwave experimenting but this is the only source of 9 cm signal in the area.

At the moment the transverter is using 90cm prime focus dish, the same one I am using for the 13cm activity with the same WA5VJB LP array. Counting the active stations in the region and the overall activity on the 9 cm band, most probably, the transverter will be boxed together with the 16dBi gain short-backfire antenna in the same w/tight housing creating a compact and portable solution for the rover activity. If you want to activate a new band or just to participate in the contest gaining your overall result this can be cheap and easy way to do it.